The present invention relates to surface-modified pigments which, for improving the settling and redispersion behavior, have been recoated with a mixture comprising two metal oxides selected from the group consisting of silica, alumina and zirconium dioxide.
Platelet-like pigments of high density are difficult to handle since the pigments show extensive settling in all liquid media used and can then lead to agglomerations with the formation of a very solid sediment cake. This cake is usually only redispersible with difficulty.
Thus, inter alia, a large number of methods have been developed in order to solve the problem of incorporation and handling of pigments in liquid compositions used.
One possibility of improving this settling behavior is to change the medium used by, for example, adding anti-settling agents or other suitable additives. One of the possibilities is, inter alia, the use of finely divided SiO.sub.2, for example aerosol. It is known from DE 3,922,178 to obtain deagglomerated and readily dispersible pigments by mixing a suspension comprising a plate-like substrate with spherical particles, such as, for example, SiO.sub.2, TiO.sub.2 or ZrO.sub.2. The disadvantage of this method is that the final result is in most cases associated with a reduction in luster.
A further possibility of improving the settling behavior is to vary the pigment surface. in DE 2,106,613 (corresponding to U.S. Pat. No. 3,650,790), mica platelets coated with metal oxides are recoated with silica, while recoating using alumina is disclosed in EP 0,446,986. Pigments coated with silicate or with Al.sub.2 O.sub.3 are difficult to disperse and additionally show losses in luster in printing inks and paint drawdowns.